A conventional technique for calculating a relative position between two vehicles based on a signal from navigation satellites is disclosed in a Japanese patent document JP-3,681,241 (also available as U.S. Pat. No. 5,999,880) for example. In the document, two vehicles respectively receive the signal from the GPS satellites for calculating signal transmission time (that is reversely proportional to a pseudo-distance) from the satellites to the vehicles. Then, one of the two vehicles, that is, the first vehicle, acquires a radio wave propagation time that is calculated on the second vehicle through an inter-vehicle communication, and the relative position of the first vehicle against the second vehicle is determined based on a difference between two radio wave propagation times on the first and second vehicles.
However, a relative position calculation method that calculates the relative position based on the difference between two radio wave propagation times derived from the signal transmission time has a calculation error ranging from several meters up to scores of meters, which is not sufficient for a specific purpose.